PURPOSE: To put the spectra of, specially, light carrier at a close interval with respect to an optical communication system which transmits two or more modulated light carriers through a single optical fiber.

CONSTITUTION: Each light carrier is modulated with a multiplex modulated subcarrier. In one example, an optical phase modulator 42 deletes the secondary intermodulation product in each light signal, and then light carrier frequencies are put at an interval of a maximum modulation frequency 2fMAX. In another example, a signal broad-band optical phase modulator deletes a secondary intermodulation product and one signal side band, and consequently the light carrier frequencies can be put just by an interval of fMAX. Therefore, the modulated light carrier frequencies can be put close in a multiplex communication, and signals can be transmitted efficiently.